Four fire engines were sent to the scene in Grove Road after West Sussex Fire and Rescue Service (WSFRS) received a call just after 9.15pm.
Crews spent several hours tackling the fire.
In a tweet, WSFRS advised people to avoid the area as crews tackled the blaze.
The fire service added that all residents at the property were safe and had been accounted for.
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Firefighters at the scene of the fire. Picture by Eddie Mitchell SUS-220704-064035001 Photo: EDDIE MITCHELL
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Firefighters at the scene of the fire. Picture by Eddie Mitchell SUS-220704-063845001 Photo: EDDIE MITCHELL
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Firefighters at the scene of the fire. Picture by Eddie Mitchell SUS-220704-063833001 Photo: EDDIE MITCHELL
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Firefighters at the scene of the fire. Picture by Eddie Mitchell SUS-220704-064046001 Photo: EDDIE MITCHELL